The present invention relates to the separation of the oxygen and nitrogen from air, and more particularly relates to gas separation means employing a novel solid state ceramic composite electrolyte.
Oxygen has a broad range of medical, industrial and experimental uses. Most of the oxygen generating apparatus provided by the prior art is voluminous and heavy due to the use of high pressure gas cylinders as the oxygen supply source.
In recent years, there have been attempts to provide compact and lightweight oxygen generating systems that can supply oxygen gas for extended periods. Japanese Utility Model Publication No. 26445/1980 discloses an oxygen gas generating system adapted to catalytically decompose aqueous hydrogen peroxide using a manganese compound as the catalyst. This system has several drawbacks. The decomposition reaction of aqueous hydrogen peroxide and manganese dioxide proceeds at an explosively high rate if the volume of hydrogen periodixe is not carefully controlled. If the volume and rate of the hydrogen peroxide reservoir is decreased to make the unit portable, the hydrogen peroxide is rapidly consumed and the reservoir must be replaced frequently. For both reasons, this is not a practical approach.
Japanese Patent Publication No. 42115/1977 employs a platinum catalyst capably of decomposing aqueous hydrogen peroxide at a high concentration. This system is also unsatisfactory, both because it requires a reservoir of hydrogen peroxide which must be periodically replaced, and because of the expense and nature of the precious metal catalyst. One problem with this approach is that the usual pore size of the alumina or silica gel catalyst support is too small to permit penetration of the hydrogen peroxide. A major drawback is that the expensive catalyst has a limited life. A further drawback is the precise temperature control required.
One attempt to address the problems with hydrogen peroxide based oxygen generating systems is disclosed in Japanese Patent Publication No. 49843/1981 in which a system is provided for controlling the flow rate of hydrogen peroxide by valve adjustment using a link mechanism to control the supply of aqueous hydrogen peroxide depending upon the pressure of the generated oxygen gas. However, the proposed system for converting the gas pressure into mechanical displacement and transmitting the displacement by means of the link has the drawback of being unable to rapidly respond to the change in the reaction rate with resulting failures due to corrosion and abrasion in the actuating system.
U.S. Pat. No. 4,792,435 discloses a system for producing oxygen by catalytic decomposition of aqueous hydrogen peroxide employing a platinum group catalyst carried on a highly porous sintered ceramic support of large pore size. This system again suffers from the drawback of requiring a hydrogen peroxide reservoir which must be periodically recharged or replaced.
U.S. Pat. No. 4,784,765 provides an aquarium oxygen generator comprising a container inverted into the apex of a ceramic cone-shaped ceramic structure resting on the floor of an aquarium. Hydrogen peroxide solution (15%) in the container is decomposed to form oxygen and water in the presence of a catalyst pellet of finely divided silver admixed with clay. Hydrogen peroxide seeps into the cone, and in the absence of the catalyst, reacts with organic material in the water to produce oxygen which bubbles through an aperture in the side of the coneshaped structure into the main body of water in the aquarium. While this system may be satisfactory for a small scale aquarium, it suffers from the drawback of requiring a hydrogen peroxide reservoir and is not suitable for medical, industrial and experimental uses.
The present invention solves the problems of the prior art and provides a system which generates oxygen or nitrogen from air, can be scaled up or down in size depending upon use, does not require consumables such as hydrogen peroxide or catalysts which must be replaced, and which is efficient and cost effective. The system of the present invention is based on a novel, flexible and mechanically rugged, thin, solid state electrolyte ceramic composite.
Ceramics generally possess a number of desirable properties, including markedly high resistance to abrasion, heat and corrosion compared to metallic materials. Certain ceramics, such as stabilized bismuth solid oxides, stabilized ceria solid oxides and zirconia solid oxides are ionically conductive materials suitable for use as solid electrolytes. However, due to extreme brittleness, their application has been limited despite their other excellent properties.
A number of attempts have been made to increase toughness of ceramic materials by compounding them with another material including ceramic or metal whiskers such as silicon carbide whiskers. Composites with ceramic matrices and ductile metal inclusions such as those produced by Lanxide Corporation show high fracture toughness when compared to ordinary ceramic materials. See for example U.S. Pat. Nos. 4,824,622; 4,847,220; 4,822,759; 4,820,461; and related 4,871,008. These composites are a chaotic, generally discontinuous, random metal dispersion in a ceramic composite body. They are prepared by a slow controlled oxidation of molten aluminum to alumina oxide, leaving behind approximately 5% of the parent metal. See also C. A. Anderson et al., Ceram. Eng. Sci. Proc., 9 [7-8] pp. 621-626 (1988); and M. S. Newkirk et al., Ceram. Eng. Sci. Proc., 8 ]7-8] pp 879-885 (1987).
P. Ducheyne et al., J. Materials Science 17(1982) 595-606 discloses a bioglass composite produced by immersing premade porous fiber skeletons into molten bioglass to prepare metal fiber reinforced bioglass. These porous fiber skeletons produce random, chaotic disordered support matrices and the process is applicable only to bioglasses.
U.S. Pat. No. 4,764,488 discloses a high toughness ceramic composite of the fiber-reinforced type wherein metal fibers having the shape of triangular waves forming bent portions alternating on the opposite sides with an angle .theta. of the bent portions in a range between 60.degree. and 165.degree. and a d/H ration of between 0.025 and 0.6. While the discrete, discontinous fibers, unidirectionally anchored fiber reinforcement employed in the--488 patent improve the toughness of the ceramic, this technique does not solve the problem of crack propagation and ultimate failure.
U.S. Pat. No. 4,776,886 discloses a whisker-reinforced ceramic matrix composite comprising a principal crystal phase selected from the group consisting of anorthite, barium-stuffed cordierite and mixed cordierite/anthorite prepared by extrusion of ceramic batches comprising an extrusion vehicle and a solid component comprising essentially inorganic whiskers and powdered glass.
The novel composite employed in the practice of this invention is mechanically tough. When subjected to intentionally severe mechanical stress, such as bending a sheet in half and restraightening it, the crack that resulted was limited to the stress or fold line.
This tough, ductile solid electrolyte composite permits the construction of a gas separation device in which the only moving part is the air intake fan, and which does not require consumables such as hydrogen peroxide or catalysts requiring constant replenishment.